1. Field of the Invention
The present invention relates to beam conditioning in illumination systems and particularly to beam conditioning systems for use in stereolithography systems.
2. Discussion of the Related Art
In recent years, rapid prototyping and manufacturing (RPandM) techniques have been developed for industrial use in the fast production of three-dimensional models. In general, RPandM techniques build a three-dimensional object, layer-by-layer, from a working material utilizing a sliced data set representing cross-sections of the object to be formed. Typically an object representation is initially provided by a computer aided design (CAD) system and the representation is translated into a number of sliced data sets that are then transferred to the successive layers of the working material.
Stereolithography, the presently dominant RPandM technique, may be defined as a technique for automated fabrication of three-dimensional objects from a fluid-like material utilizing selective solidification of thin layers of the material at a working surface to form and adhere successive layers of the object (i.e., laminae). In stereolithography, data representing the three-dimensional object are input as, or converted into, two dimensional layer data representing cross-sections of the object to be formed. Thin layers of material are successively formed and selectively transformed (i.e., cured) into successive laminae according to the two-dimensional layer data. During transformation the successive laminae are bonded to previously formed laminae to allow integral formation of the three-dimensional object.
A preferred material used in a stereolithographic apparatus (SLA) is a liquid photopolymer resin. Typical resins are solidified by exposure to selected wavelengths of electromagnetic radiation (e.g. selected wavelengths of ultraviolet (UV) radiation or visible light). This radiation of selected wavelength may be termed xe2x80x9csolidifying radiation.xe2x80x9d The electromagnetic radiation is typically in the form of a laser beam that is directed to a target surface of the resin by two computer controlled scanning mirrors that scan the target surface along orthogonal directions. The scanning speed, pulse repetition frequency and spot size of the beam on the liquid surface are controlled to provide a desired exposure, depth of cure and solidification characteristics.
A more detailed description of stereolithography and the methods and apparatus for practicing photolithography are found in the following patents, which are hereby incorporated by reference:
U.S. Pat. No. 4,575,330 to Hull: Describes the fundamentals of stereolithography.
U.S. Pat. No. 5,058,988 to Spence, et al.: Describes the use of beam profiling techniques in stereolithography.
U.S. Pat. No. 5,059,021 to Spence, et al.: Describes the use of scanning system drift correction techniques for maintaining registration of exposure positions on the target surface.
U.S. Pat. No. 5,104,592 to Hull et al.: Describes the use of various scanning techniques for reducing curl-type distortion in objects that are being formed stereolithographically.
U.S. Pat. No. 5,123,734 to Spence, et al.: Describes a technique for calibrating a scanning system on a stereolithographic apparatus.
U.S. Pat. No. 5,133,987 to Spence, et al.: Describes the use of a large stationary mirror in the beam path between the scanning mirrors and a target surface.
U.S. Pat. No. 5,182,056 to Spence, et al.: Describes the simultaneous use of multiple wavelengths to expose the resin.
U.S. Pat. No. 5,184,307 to Hull, et al.: Describes the use of slicing techniques for converting three-dimensional CAD data into cross-sectional data for use in exposing the target surface to appropriate stimulation.
U.S. Pat. No. 5,321,622 to Snead, et al.: Describes the use of Boolean operations in deriving cross-sectional data from three-dimensional object data
U.S. Pat. No. 5,965,079, to Gigl, et al.: Describes various scanning techniques for use in stereolithography.
U.S. Pat. No. 5,999,184, to Smalley, et al.: Describes the use of solidification techniques to simultaneously cure multiple layers.
U.S. Pat. No. 6,129,884 to Beers, et al.: Describes the control of a pulsed illumination source to achieve desired solidification characteristics.
Commercially available photopolymer for use in stereolithography are typically of acrylate, epoxy or combined chemistry. Typically, resins contain a plurality of components. These components may include one or more photoinitiators, monomers, oligomers, inert absorbers, and other additives. The usefulness of resins for stereolithography is in part determined by the photospeed of the resin and the ability of the resin to form adequately cohesive laminae of appropriate thickness. It is desired that the photospeed be high enough to enable rapid solidification of cross-sections with available power levels of solidifying radiation. Further, since the depth of polymerization in the resin is linked to the locations at which photons are absorbed, absorption of photons by the resin should be sufficient to form adequately thin layers. Examples of preferred photopolymers include, but are not limited to, SL 7540, SL 7520, SL 7510, SL 5530, SL 5520, SL 5510 and SL 5195 (manufactured by Vantico, Inc. and as sold by 3D Systems, Inc. of Valencia, Calif.), SOMOS 9120, 9100, 8120, 8100, 7120 and 7120 (manufactured by DSM Somos of New Castle, Del.).
Photoinitiators are the component of the resin that determines the photosensitivity of the resin at a given wavelength. Radiation absorption by the photoinitiator leads to chemical changes in the photoinitiator that can cause polymerization of the monomers and oligomers. Thus, radiation of appropriate wavelengths to be absorbed by the photoinitiator is known as solidifying radiation. The monomers/oligomers can absorb certain wavelengths of electromagnetic radiation. As absorption by the monomers/oligomers typically does not yield an efficient polymerization reaction, absorption of solidifying radiation by the monomers/oligomers is typically undesired. Thus, the most effective wavelengths for use in stereolithography are those strongly absorbed by the photoinitiator (high coefficient of absorption) and only weakly absorbed by the monomers and oligomers (low coefficient of absorption). Examples of preferred photoinitiators include, but are no limited to, triarylsulfonium salts, mixtures of triarylsulfonium salts with phosphate salts or antimonate salts; 2,2-dimethoxy-2-phenyl acetophenone (BDK) C16H16O16; 2,4,6-trimethyl benzoyl diphenyl phosphine oxide (TPO); an 1-hydroxycyclohexyl phenyl ketone (HCPK) C13H16O2.
The useable wavelength range is bounded at the low wavelength end by monomer/oligomer absorption properties and at the upper wavelength end by photoinitiator absorption. As such, the reactive (i.e., actinic) spectral sensitivity of a photopolymer resin may be described as the product of the photoinitiator absorption spectrum and the monomer/oligomer transmission spectrum, as shown in FIG. 1. Note that th FIG. 1 illustration is for a particular photopolymer system. Other systems exist and will have different curves, providing different optimal illumination sources. FIG. 1 depicts plots of photoinitiator absorption 11, monomer/oligomer transmission 13, and reactive sensitivity or reactive response 15 of the resin. The absorption and transmission coefficients not only depend or the specific chemical composition of each component, but also on the concentrations of each component within the resin. The absorption by the monomer/oligomer, which depends upon the wavelength of radiation, affects the activation of the photopolymers because the monomer/oligomer absorption sometimes competes with the photoinitiator absorption. Consequently, shifts in wavelength for peak reactive response may result due to changes in either composition or concentration. For a given resin composition this peak can be readily determined by one of skill in the art. Those of ordinary skill appreciate that different light sources require use of different resin compositions.
In the example of FIG. 1, the peak reactive response occurs within a range of about 328 nm-337 nm and the half-maximum response falls within the range of about 320 nm to about 345 nm. As such, in this example electromagnetic radiation within the range of 320 to 345 nm is preferred and electromagnetic radiation within the range of 328 to 337 nm is even more preferred. The more preferred range include the wavelengths within 10% of the peak reactive response. The preferred range includes wavelengths within 50% of the peak reactive response. For different resin systems and response curves, different preferred ranges can be established in the same manner.
Until recently, commercial stereolithography systems used helium-cadmium (HeCd) lasers that emit radiation a 325 nm or argon-ion lasers that emit radiation primarily at 351 nm. Helium-cadmium lasers have a wavelength, input power and output that are suitable and acceptable for stereolithography. The output power from HeCd lasers is undesirably limited and unsuitable for building large objects or when faster build speeds are needed. Thus, although HeCd lasers are useful in stereolithography, they do not achieve all of the needs of stereolithography.
Argon-ion lasers have output power levels and output modes suitable for faster part building and/or larger stereolithography parts. On the other hand, the input power is excessive, and necessitate water-cooling.
Present diode pumped solid state (DPSS) lasers have both input and output powers suitable for stereolithography. These solid state lasers are pulsed where the prior gas lasers (e.g., HeCd nd Ar+) provide a continuous wave (CW) laser beam. To effectively utilize these solid state lasers a sufficiently high pulse repetition rate is needed to ensure that continuous cured lines of photopolymer are formed.
Recent commercial stereolithographic systems have employed pulsed solid state lasers to selectively solidify the material. These commercial systems frequency triple the 1064 nm fundamental infrared radiation of Nd:YVO4 pulsed solid state lasers to generate ultraviolet output light. Frequency tripling generates an output wavelength of 355 nm. Resins appropriate to use with 355 nm light sources are known and commercially available.
An aspect of the present invention provides an optical system including spot size control optics and focus control optics. The spot size control optics receives a beam of light, adjusts a lateral extent of the beam of light and outputs the beam of light. The beam of light has an extent in a first lateral direction and a second lateral direction perpendicular to the first lateral direction. The spot size control optics are coupled to an actuator responsive to electrical signals to adjust an ellipticity of the beam, wherein movement of the actuator alters the first lateral extent of the beam of light more than the second lateral extent. The focus control optics receives the beam of light, alters a position of a focus of the beam of light and outputs the beam of light.
Another aspect of the present invention provides an optical system including spot size control optics and focus control optics. The spot size control optics receive a beam of light, adjust a lateral extent of the beam of light and output the beam of light. The system includes focus control optics coupled to an actuator responsive to electrical signals. The focus control optics receives the beam of light, alters a position of a focus of the beam of light and outputs the beam of light. The beam of light has a first focus position for a first lateral component of the beam of light and has a second focus position for a second lateral component of the beam of light. The first lateral component is selected to be perpendicular to the second lateral component. The focus control optics adjusts the first focus position to a greater extent than the second focus position in response to the electrical signals supplied to the actuator.
Still another aspect of the present invention provides an optical system including spot size control optics and focus control optics. The spot size control optics receives a beam of light, adjusts a lateral extent of the beam of light and outputs the beam of light. The beam of light has an extent in a first lateral direction and a second lateral direction perpendicular to the first lateral direction. The spot size control optics are coupled to a spot size actuator responsive to electrical signals to adjust an ellipticity of the beam, wherein movement of the spot size actuator alters the first lateral extent of the beam of light more than the second lateral extent. The system includes focus control optics coupled to a focus actuator responsive to electrical signals. The focus control optics receives the beam of light, alters a position of a focus of the beam of light and outputs the beam of light. The beam of light has a first focus position for a first lateral component of the beam of light and has a second focus position for a second lateral component of the beam of light. The first lateral component is selected to be perpendicular to the second lateral component. The focus control optics adjust the first focus position to a greater extent than the second focus position in response to the electrical signals supplied to the focus actuator.
Yet another aspect of the present invention provides an optical system including a laser system, spot size control optics, focus control optics and beam positioning optics. The laser system includes a solid state laser and outputs a beam of light to the spot size control optics, which adjusts the lateral extent of the beam of light and outputs the beam of light. The spot size control optics include a first lens mounted on a pivot and a linear translation stage. A first actuator is coupled to rotate the first lens on the pivot and a second actuator is coupled to translate the lens along the linear translation stage. The first and second actuators are responsive to electrical signals. The focus control optics receive the beam of light adjusted by the spot size control optics, alters a position of a focus of the beam of light and outputs the beam of light. Beam positioning optics receive the beam of light altered by the focus control optics and laterally position the beam of light.
Another aspect of the invention provide an optical system having a laser system outputting a beam of light to spot size control optics that receives the beam of light, adjusts a lateral extent of the beam of light and outputs the beam of light. Focus control optics receives the beam of light adjusted by the spot size control optics, alters a position of a focus of the beam of light and outputs the beam of light. The focus control optics include a first lens mounted on a pivot and a linear translation stage, a first actuator coupled to rotate the first lens on the pivot and a second actuator coupled to translate the first lens along the linear translation stage. The first and second actuators are responsive to electrical signals. Beam positioning optics receives the beam of light altered by the focus control optics and laterally position the beam of light.